The "KNOWN universe" is defined by the greatest distance to which we can see with our current technology. It is a radius and associated with that radius is the geometric notion of a circumference. It is that circumference to which the quote refers, not the as yet unknown boundary of the universe, if indeed one exists.
The most puzzling issue to me revolves around the fact that the universe is expanding. Into what is it expanding ? The question is as mind contorting as speculation of the origin of the big bang.
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